The present invention relates generally to instrument calibration and more particularly to a novel method and apparatus for calibration of indirect measurement and inspection systems.
Inspection and measurement systems often depend on accurately sensing one or more physical parameters reflecting a process of interest. In electronics, for example, automated test equipment (ATE) may, in various situations, be required to accurately measure voltage, current, resistance, or phase. In radiation dosimetry, systems may be required to accurately measure total accumulated dose or radiation level. Similarly, systems may be required to measure length, time, mass, or any of a variety of physical parameters. It is generally desirable that the measured result be repeatable, portable, and accurate. Repeatability means that multiple consecutive readings of the same parameter on a single system give the same result within the required tolerance. Repeatability is primarily a function of system design and is beyond the scope of the present invention. Portability means that comparable results should be obtained if the same measurement is taken using different instances of the same system (i.e., two voltmeters of the same model should return similar or identical results when performing the same measurement). Accuracy means that the measured results should be a true reflection of the underlying process, without the introduction of bias. Since all measurement and inspection systems are subject, in varying degree, to manufacturing variations and drift, periodic calibrations of the measurement system are typically required to assure portability and accuracy.
Frequently, calibration involves adjusting the system to obtain a correct reading on one or more known calibration samples. It is desirable and often required, for the calibration samples to be traceable to a primary standard maintained by a widely recognized agency. In the United States, for example, the National Institute of Standards and Technology (NIST) typically maintains primary calibration standards. Depending on the economics of a particular situation, calibration samples may be shipped with the system or may be incorporated into the system itself. More commonly, however, small systems are returned to centralized metrology labs for periodic calibration, while for larger systems it may be necessary to transport the standards to the system. Such approaches are widely used and often suffice when the physical property of interest is directly sensed by the system in question, or when the relationship between the quantity sensed and the physical property of interest is well known. (An accurate voltage sensor can be combined with a known resistance to provide a current sensor, for example).
In many cases, however, inspection and measurement systems must rely on measurements that only indirectly reflect the physical parameters of interest. The Agilent 5DX Automated X-ray Inspection (AXI) System manufactured by Agilent Technologies Inc. of Palo Alto, Calif., for example, uses penetrating radiation, specifically x-rays, to form cross-sectional images of solder joints in electronic assemblies, and to automatically identify defective and/or unreliable joints. A key physical parameter for such identification is solder thickness. Unfortunately, the relationship between solder thickness and the gray value observed in cross-sectional or transmission images is complex and difficult to model accurately. While thicker joints typically appear darker, all other factors being equal, the relationship is complicated by several factors. Due to scatter, the gray value observed for a particular joint can be affected by neighboring or even distant portions of the assembly. Additional problems arise from the use of broadband (typically bremsstrahlung) x-ray sources in conjunction with monochromatic detectors. When the object under inspection comprises multiple materials, each material has an x-ray attenuation coefficient that depends on energy in a characteristic way. Monochromatic detectors sense only the integrated intensity reaching the detector, so it is typically not possible to unambiguously recover the thickness of the intervening materials. Similar problems are well known in other areas, for example, in various forms of quantitative medical imaging including, but not limited to, computed tomography (CT), positron emission tomography (PET), and single photon positron emission computed tomography (SPECT).
Calibration in such indirect-sensing systems is often expensive, time-consuming, and error prone. A calibration coupon, for example, comprising differing known values of a parameter of interest may be provided with each indirect-sensing system to be used as a set of calibration samples. For calibration, measurements are taken of an indirect parameter under standard conditions, and a fitting procedure generates a mathematical model describing the non-linear relation between the measured indirect parameter value and the actual parameter of interest. The fitting procedure must be repeated periodically to compensate for drift of known and unknown origin.
As a specific example, consider automated X-ray inspection systems such as the Agilent 5DX AXI System mentioned previously. Using the prior art technique, each 5DX system site is provided with a thickness coupon containing eight copper strips of varying thickness (including a thickness of zero) which intersect nine solder strips of varying thickness (including a thickness of zero), for a total of seventy-two “representative” calibration samples. During calibration, the gray values of each intersection point are measured under standard conditions, and a fitting procedure, such as one described in U.S. Pat. No. 6,201,850 (hereby incorporated by reference for all that it teaches) to Heumann and owned by the present assignee of interest, constructs a mathematical model describing the non-linear relation between measured gray values and corresponding solder thicknesses.
A number of disadvantages, representative of indirect measuring systems, are apparent in the above example. A large number of calibration samples may be required, resulting in large expenses. Additionally, measuring many samples accurately is time consuming, reducing system availability for normal use. Fabrication of the calibration samples can pose its own challenges. In the case of a thickness coupon, described above, eutectic solder is soft and easily deformed. As a result, strips whose thickness is controlled to the desired accuracy are not readily available commercially. This necessitates measuring the thickness of each solder strip using a NIST traceable instrument. However, mechanical measurement can alter solder strip thickness. Thickness may therefore not be known precisely, and may vary within a coupon or from coupon to coupon. Thickness measurements may suffer from portability and accuracy problems as a result. Finally, the need to minimize the time that the system is unavailable for production use limits the sophistication of the fitting procedure used to describe the relationship between measured gray values and solder thickness.
Accordingly, a calibration technique for indirect measurement and inspection systems is needed that avoids the aforementioned problems, yet is economical, automated, fast, highly repeatable, portable, and accurate.